Fluid cylinder having self-locking means

ABSTRACT

A fluid cylinder having a tubular housing with a piston reciprocably mounted therein and capable of fluid pressure actuation in opposite directions. One side of the piston carries an actuating rod which slidably extends through one end of the tubular housing, while a coaxially aligned bore extending from the opposite side of the piston and into the actuating rod has a threaded surface adapted to receive a threaded member which, in turn, is rotatably carried at the other end of the tubular housing. The threaded surfaces of the piston bore and the rotatable threaded member engage in a self-locking relationship such that the piston is incapable of movement when subjected to fluid pressure unless the threaded member is initially and continually rotated. Means are provided for selectively rotating the threaded member upon fluid pressure actuation of the piston in either direction.

Unite States Patent 1191 Emenaker 1 May 15, 1973 54 FLUID CYLINDERHAVING SELF- 3,550,731 12 1970 Roselius ..91/41 LOCKING M N 2,859,734 111958 Elmer et al. ..91/45 3,472,124 10/1969 Roselius et al. ..91/45 [75]Inventor: Richard G. Emenaker, Romeo,

Mich" Primary Examiner-Paul E. Maslousky [73] Assignee: Harry S.Nichols, Jr., Troy, Mich. An And w R, Basile t t a par mews 57 ABSTRACT[22] Filed 1971 A fluid c linder having a tubular housin with a iston yg P [2]] Appl. No.: 114,575 reciprocably mounted therein and capable offluid pressure actuation in opposite directions. One side of the istoncarries an actuatin rod which slidabl ex- 52 us. (:1. ..91 44,92 17,9233 P g Y Int. Cl 0 I I I I I F/lsb 4 tends through one end of thetubular housing, while a [58] Field of 7 44 coaxially aligned boreextending from the opposite side of the piston and into the actuatingrod has a threaded surface adapted to receive a threaded [56] ReferencesCited member which, in turn, is rotatably carried at the other end ofthe tubular housing. The threaded sur- UNITED STATES PATENTS faces ofthe piston bore and the rotatable threaded member engage in aself-locking relationship such that ga g the piston is incapable ofmovement when subjected 3218937 11/1965 to fluid pressure unless thethreaded member is ini- 21918786 12/1959 Geyer ..91/45 tially andcontinually rotated- Means are Provided for 2,688,227 9 1954 Geyer ..9217 Selectively rotating the threaded member p fluid 2,660,028 11/1953Geyer.... ..91/45 pressure actuation of the piston in either direction.2,660,027 11/1953 Geyer ..91/45 7 Claims, 3 Drawing Figures PATENIEB'M'H $132,783

INVENTOR W RICHARD G.EMENAKER FLUID CYLINDER HAVING SELF-LOCKING MEANSBACKGROUND OF THE INVENTION 1. Field of the Invention The presentinvention relates to fluid pressure operated actuators and particularlyto a linear fluid cylinder actuator having self-locking means.

2. Description of the Prior Art Heretofore in the design of fluidsystems, numerous mechanisms have been employed to prevent the movementof a piston within a fluid cylinder in the event of a failure in thefluid system. One such system employs complicated valving mechanisms andcircuitry which are adapted to prevent the exhausting of fluid from thatside of the piston which is opposing the external load. Although suchsystems have functioned in an acceptable manner, resistance to themovement of the piston, and thus to the external load, in such systemsdiminishes with increasing use as the valves employed in the systemstend to develop volumetric losses that may go undetected and whicheventually results in the movement of the piston. In other situationswhen the system failure is due to a faulty piston seal, that is, fluidleakage occurs directly between the opposite sides of the piston withinthe fluid cylinder, most valve systems are completely useless and thepiston will be moved by the load. In many applications this movement mayresult in damage not only to the fluid cylinder and the equipment beingoperated on, but may cause injury to the operator of the fluid cylinder,as for example when such fluid cylinders are employed for use on ahoist.

Other mechanisms have been employed in combination with fluid cylindersin order to control the movement of the piston within the fluidcylinder. One such mechanism comprises a fluid cylinder having areciprocably mounted piston with an actuating rod extending from oneend, while the other end of the piston has a threaded bore adapted toreceive a threaded rotating member. The other end of the rotating memberextends externally of the fluid cylinder and is adapted to be engaged bya suitable locking device to prevent relative rotation between themember and the piston. When the threaded member is restrained againstrotation, the piston, which likewise is restrained from rotation,is'incapable of movement in an axial direction. Such prior. art systemshave the same disadvantages as the aforementioned fluid systems in thatthey are dependent upon an external locking means and an external sourceof power to initiate and maintain restrainment in the movement of thepiston when an external load is applied to the piston.

SUMMARY OF THE INVENTION The present invention, which will be describedsubsequently in greater detail, comprises a fluid cylinder having apiston reciprocably mounted therein for axial movement in response tofluid pressure actuation, the piston having a threaded bore receiving arotatably mounted threaded member with the engaged surfaces of thepiston bore and threaded member being in a selflocking relationship suchthat the piston is nonreciprocal when subjected to fluid pressure unlesssaid threaded member is simultaneously rotated in said piston bore.

It is therefore an object of the present invention to provide a fluidcylinder having a piston adapted to be selectively restrained fromreciprocal movement at any point in its stroke.

It is a further object of the present invention to provide a fluidcylinder having self-locking means for automatically stopping themovement of the cylinder piston in the event of a system failure, whichlocking means is also adapted to function as a back-up system fordriving said piston.

It is a further object of the present invention to provide a fluidcylinder having locking means which function without the need for anexternal source of power.

Other objects, advantages, and applications of the present inventionwill become apparent to those skilled in the art of fluid cylinders whenthe accompanying description of a preferred embodiment for practicingthe invention is read in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING The description herein makes referenceto the accompanying drawing wherein like reference numerals refer tolike parts throughout the several views, and in which:

FIG. 1 is a longitudinal cross-sectional view of a fluid cylinderincorporating the inventive principles of the present invention;

FIG. 2 is a transverse cross-sectional view of the fluid cylinder takenalong line 2-2 of FIG. 1; and

FIG. 3 is a schematic circuit diagram of a fluid system adapted to beused in conjunction with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. I, a fluidcylinder 10 is illustrated as comprising a tubular housing 12 enclosedat its opposite ends by end covers 14 and 16, each of which haverecesses 18 which receive the opposite ends of the tubular housing 12.The end covers 14 and 16 are secured to the tubular housing 12 by aplurality of elongated bolts 19 which extend through the end covers 14and 16 to engage nuts 20. Conventional O-rings 22 are provided inannular recesses 24 at the juncture of the end covers with the outerperipheral surface of the tubular housing 12 to provide a suitable seal.

The fluid cylinder 10 has a piston 26 reciprocally mounted within thetubular housing 12, the piston 26 dividing the interior of the tubularhousing into an extending chamber 28 and a retracting chamber 30. Thepiston 26 includes a suitable sealing means, such as an O-ring 34,disposed in an annular recess 36 extending around the outer periphery ofthe piston 26 for sliding engagement with the inner surface of thetubular housing 12. The piston 26 is further provided with a hollowactuating rod 38, enclosed at its outer end (not shown), and whichextends from one side of the piston 26 through a bore 40 in the endcover 14 for connection at its outer end to a movable load (not shown).The actuating rod 38 is adapted to .be reciprocated with the piston 26along the axis of the tubular housing 12 in a manner to be described ingreater detail hereinafter. O-rings 42, disposed in annular recesses 44formed in the end cover bore 40, provide a suitable seal to prevent theleakage of fluid from the retracting chamber 30. The actuating rod 38has a threaded inner end 46 which engages a centrally disposed threadedbore 48 in piston 26 to provide a suitable attachment of the rod 38 topiston 26, however, otherattaching means, such as a press-fit orwelding, may be employed to attach the rod 38 to the piston.

A portion of the interior 49 of the hollow actuating rod 38 has athreaded surface 52 commencing from the extending chamber end of theactuating rod 38 and terminating at the enclosed end of rod 38 or atsome selected distance therefrom. The threaded surface 52 of theactuating rod 38 is engaged by a mating threaded portion 55 of arotatably mounted shaft 57. The shaft 57 has a length approximatelyequal to the length of the tubular housing 12, while the threadedportion 55 extends substantially the full length of the shaft 57. Theinterior 49 of the actuating rod 38 may be vented back to the extendingchamber 28 by any suitable means, such as axial and radial bores 150extending through shaft 57.

The retracting chamber 30 is adapted to be selectively connected to asource of fluid pressure or a fluid reservoir through a passageway 56and a fluid port 58, both of which are located in the end cover 14,while the extending chamber 28 is adapted to be selectively connected toa source of pressure or a fluid reservoir through a passageway 60 and afluid port 62, both of which are located in the end cover-16.Pressurization of the retracting chamber 30 generates a force on thepiston 26 which tends to move the same rightwardly, as viewed in FIG. 1,while fluid is exhausted fromthe extending chamber 28 and pressurizationof the extending chamber 28 generates a force on the piston 26 whichtends to move the same leftwardly, as viewed in FIG. 1, while fluid isexhausted from the retracting chamber 30.

Still referring to FIG. 1, the fluid cylinder is further provided withan elongated rod 64 engaged at its opposite ends within apertures 66formed in the interior walls of the end covers 14 and 16, the rod 64extending through an axial bore 68 in the piston 26. The rod 64 permitsthe piston 26 to reciprocatein an axial direction within the tubularhousing 12, while restraining the piston 26 from rotation about thelongitudinal axis of the tubular 12, however, other means may beemployed, such as external means, to restrain the piston 26 fromrotating. Suitable seals, such as O-rings 54, are provided in the bore68 to prevent fluid communication between the opposite side of thepiston 26.

The end cover 16 is comprised of two sections, an inner block 70 inwhich the passageway 60 and fluid port 62 are located, and an outerblock72 in which a rotary actuator 74 is located. The inner wall of the innerblock 70 engages the outer end of the tubular housing 12, while theinner wall of the outer block 72 engages the outer wall of the innerblock 70 when the elongated bolts 19 are secured by the nuts 20.Suitable sealing means, such as an O-ring 76, disposed in an annularrecess 78 in the inner wall of the outer block 72, forms a fluid sealbetween the juncture of the inner and outer blocks.

The inner block 70 has a centrally disposed bore 80 that is axiallyaligned with the piston 26 and the actuating rod 38. The unthreadedportion of the shaft 57 extends through the bore 80 and is rotatablysupported by a radial-thrust bearing 82 which, in turn, is mounted in arecess 84 in the outer wall of the inner block 70. The bearing 82 is inabutment with shoulder 86 on the shaft 57 to receive axial loads imposedin a leftwardly direction, as viewed in FIG. 1. An O-ring 88, disposedin an annular recess 90 circumscribing the shaft 57, provides a suitableseal to prevent leakage from the extending chamber 28. Referring to bothFIGS. 1 and 2, the shaft 57 is shown as having a radially enlargedsection 92 with a pair of spline sections 94 that are adapted to engageenlarged diametrically opposed slots 95 in a rotor 96 for rotationtherewith about the axis of the shaft 57. The end of the shaft 57 isrotatably supported by a second radial-thrust bearing 98 which isdisposed in a blind recess 100 in the outer block 72. The bearing 98 isin abutment with'a shoulder 102 on the enlarged section 92 and isadapted to receive axial loads transmitted through the shaft 57, thatis, those forces imposed in a rightwardly direction, as viewed in FIG.1.

The rotor 96 is adapted to rotate in a cup-shaped recess 104 in theinner wall of the outer block 72 which in conjunction with the adjacentwall of the inner block 70 forms a circular chamber 106 havingtangentially disposed fluid ports 108 and 110 (FIG. 2). Although notshown, a small clearance is provided between the opposite outer sides ofthe rotor 96 and their respective opposing faces of the block 70 and 72.The rotor 96 has a peripheral recess 112 which is divided into aplurality of sub-chambers .113 by a plurality of vane members -114carried by the walls defining the rotor recess 112. The vane members 114 may be attached to the rotor 96 by any suitable means such as beingintegrally formed with the rotor 96, as shown. When'fluid under pressureis admitted to each sub-chamber 113 through the fluid port 108, theforce of the fluid striking the vane members 114 will cause the rotor 96to freely rotate in a clockwise direction, as viewed in FIG. 2, untilthe walls 116 of the rotor slots 95 will impact .the spline sections 94.The continued flow of fluid against the vane members 114, in addition tothe momentum of the rotor 96, will rotate the shaft 57. As the rotor 96rotates in a clockwise direction, the fluid is exhausted through thefluid port 110. In the same manner, fluid under pressure admitted to thesub-chambers 113 through the fluid port 110 will cause the rotor andthus shaft 57 to rotate in a counterclockwise direction, as viewed inFIG. 2, while fluid is exhausted through fluid port 108.

It should be noted that any axial loads which are transmitted from theactuating rod 38 and the piston 26 are taken by the bearings 82 and 98as the shoulders 86 and 102 on the opposite sides of the enlargedsection 92 are in an abutting force transmitting relationship with thebearings and not with the rotor 96,.Thus, the rotor 96 is not subjectedto these axial loads.

The shaft 57 and actuating rod 38 each may be constructed of anysuitable material depending upon the desired application, while themating threaded portions 52 and 54 of the actuating rod 38 and the shaft57, respectively, may be of any desired configuration. In the preferredembodiment, the threaded portions 52 and 54 are illustrated as having aconventional Unified Thread of a predetermined pressure angle A and alead angle B. In the present example of the invention, the relationshipbetween the threaded portion 55 of the shaft 57 and the threaded portion52 of the actuating rod 38 is such that the two mating threaded portionsengage in a self-locking manner and thus there can be no axial movementof the piston 26 within the tubular housing 12 when pressure is actingon the piston 26 to move the same as hereinbefore described, unlessthere is first relative rotational movement between the piston 26 andthe shaft 57. Since the piston 26 and the actuating rod 38 arerestrained from rotating by the rod 64, the shaft 57 must be rotated topermit axial movement of the piston 26 and rod 38.

In the embodiment illustrated, a self-locking relationship is achievedwhen the coefficient of friction between the engaged threaded portion 55of the shaft 57 and the threaded portion 52 of the actuating rod 38 isat least equal to or greater than the product of the cosine of thepressure angle A and the tangent of the lead angle B, that is:

Coefficient of Friction (cos A) (Tan B) When this relationship exists,the piston 26 will not reciprocate within the tubular housing 12,irrespective of how much pressure is admitted to either the extending orretracting chambers 28 and 30, until the shaft 57 is rotated relative tothe actuating rod 38 and the piston 26.

Referring to FIG. 3, there is illustrated a schematic circuit diagram ofa preferred hydraulic system 120 adapted for particular use with thepresent invention and comprising a pump 122 adapted to draw fluid from areservoir 124 through a fluid filter 126 and a conduit 128. The pump 122directs pressurized fluid through conduits 130 and 132 to a pair ofconventional solenoid actuated directional control valves 134 and 136,respectively. The control valve 134 is adapted to selectively directfluid under pressure to one of the fluid ports 58 or 62 on the oppositesides of the fluid cylinder through conduits 138 and 140, respectively,for selectively pressurizing either the retracting chamber 30 or theextending chamber 28 to thereby cause reciprocation of the piston 26 ashereinbefore explained. An exhaust conduit 142, connected to the valve134, communicates the unpressurized chamber to the reservoir- 124. Atthe same time the second control valve 136 may selectively direct fluidunder pressure from pump 122 to one of the fluid ports 108 or 110 of therotary actuator 74 through conduits 144 or 146, while fluid beingdischarged from the other port 108 or 110 of the actuator 74 isexhausted through exhaust conduit 142.

In operation, when extension of the actuating rod 38 is desired,pressure fluid is directed through fluid port 62 to the extendingchamber 28, while simultaneously pressure fluid is directed to the fluidport 110 of the rotary actuator 74 so as to rotate the shaft 57 in acounterclockwise direction (as viewed in FIG. 2). In normal operation,the rate at which the shaft 57 is rotated determines the speed at whichthe piston 26 will traverse within the tubular housing 12; however, itis the pressure within chamber 28 acting on the piston 26 thatdetermines the force imparted to the load. Although, the main purposefor rotating the shaft 57 is to permit axial movement of the piston 26,the shaft 57 does provide an additional force to aid in the movement ofthe piston 26. When it is desired to retract the piston 26, the positionof directional control valve 134 and 136 are reversed to direct pressurefluid through fluid port 58 to the retracting chamber 30 and to thefluid port 108, respectively, to exert a force on piston 26 to move thesame rightwardly as the rotary actuator 74 is rotated in an oppositedirection, that is, in a clockwise direction as viewed in FIG. 2.

It will be noted that although the preferred embodiment illustrates animpulse type rotary actuator 74 for causing relative rotational movementbetween. the

threaded portions 52 and 55 of the actuating rod 38 and the shaft 57respectively, other actuators, such as a gear motor, a helical spline ora piston-rack actuator, may be employed to accomplish the same results.

It can also be seen that at any time during the movement of the piston26 with respect to the tubular housing 12 a cessation in the flow of thepressure fluid to the rotary actuator 74 will immediately stop themovement of the piston 26 as the piston cannot move due to the lockingrelationship between the engaged mating threaded portions 52 and 55. Inaddition to the capability of stopping the piston 26 at any point alongits path of movement between an extended and a retracted po sition, theneed for a cushion, as is typical in such fluid cylinders, is notrequired as the locking engaging between the piston 26 and the shaft 57can function in place of a cushion to prevent impact of the piston 26with the end covers 14 and 16.

It should also be noted that a cessation in the move ment of the piston26 as produced by the non-rotation of the shaft 57 prevents the piston26 from moving irrespective of the amount of pressure generated ineither of the retracting or extending chambers and thus the piston 26can be restrained against axial movement without any external source ofpower being applied to the shaft 57. a v

In the event of a failure in the system 120, either because of ablow-out across the piston 26 between the retracting and extendingchambers or a failure in the conduits connecting the chambers to thepump 122, the piston 26 may be immediately stopped in position and theload restrained against further movement by rotary actuator 74 has thecapacity to impart a reciprocal movement to the actuating rod 38 throughthe engagement of the threaded portions 52 and 55.

In the event of a system failure with respect to supplying pressurefluid to the rotary actuator 74, such as a failure of pump 122, the.rotary actuator 74 will not rotate and thus the piston 26 is-restrainedfrom axial movement by the locking engagement between the threadedsurfaces 52 and 55. In this condition, the external load will beprevented from moving.

It can thus be seen that the present invention has provided a new andimproved fluid cylinder which is simple in its construction; whichfunctions as a safety lock in the event of a pressure failure withoutthe need of an external power source, and is adapted to function as aback-upsystem to provide a driving force for the fluid cylinder.

What is claimed is as follows: 1. A fluid pressure operated actuatorcomprising: a tubular housing having an internal bor enclosed at itsopposite ends; I

piston means axially reciprocally disposed in bore tubular housing boreand dividing said bore into two pressure chambers;

means for communicating pressure fluid to one of said pressure chamberswhile exhausting the other of said pressure chambers to move said pistonmeans in one direction in said tubular housing bore;

an actuating rod carried by said piston means and extending from oneside of said piston means through one enclosed end of said tubularhousing;

a member extending from the opposite side of said piston means androtatably supported inthe other enclosed end of said tubular housing,said member having a threaded portion engaging a mating threaded portionof the actuating rod carried by said piston means, the engaging threadedportion carried by said piston means and the threaded portion of saidmember being in a self-locking relationship such that said piston meansis axially movable only when said one pressure chamber is communicatedto said pressure fluid and said member is rotated relative to saidpiston means;

non-locking means for rotating said member relative to said pistonmeans, said non-locking means being carried by said other enclosed endof said tubular housing; and

means coupling said non-locking means to said member such that saidnon-locking means and said member are rotatable together after a limitedamount of independent rotational movement of said non-locking meansrelative to said member;

said non-locking means for rotating said member relative to said pistonmeans comprising a rotor carried by said member, an enclosed chamberformed in said other enclosed end of said tubular housing, said rotorhving a plurality of radially extending vane members which divide saidlast mentioned enclosed chamber into a plurality of subchambers; and apair of ports, one of which is adapted to direct pressure fluid intosaid subchambers against said vane members to cause rotation of saidrotor and thus said member in one angular direction, the other saidports being adapted to exhaust fluid from said subchambers.

2. The fluid pressure operated actuator defined in claim 1 furthercomprising means for preventing relative rotation between said pistonmeans and said tubular housing.

3. The fluid pressure operated actuator defined in claim 1 wherein saidthreaded portion carried by said piston means comprises a bore extendingalong the longitudinal axis of said actuating rod and opening to saidopposite side of said piston means, a portion of said bore beingthreaded a selected distance, said rotatably supported member being inaxial alignment with said last-mentioned'bore and engaged therein.

4. The fluid pressure operated actuator defined in claim 3 wherein theproduct of the cosine of the pressure angle of the member and thetangent of the lead angle of the member is equal to or greater than thecoefficient of friction of said engaged threaded portions.

5. The fluid pressure operated actuator defined in claim 1 wherein saidcoupling means comprises:

a circular section formed on said member, said circular section havingat least one radially extending motion transmitting portion, said rotorhaving a circular bore receiving said circular section such that saidrotor is angularly slidable about said circular section, said rotorhaving a recessed portion opening to said rotor circular bore, saidrecessed portion receiving saidmotion transmitting portion, saidrecessed portion being so sized with respect to said motion transmittingportion to permit said rotor to rotate about said circular section alimited angular distance prior to engagement of a wall of said recessedportion with said motion transmitting portion as said rotor is rotatedwhen one of said pair of ports is communicated to said fluid pressure,said member being rotated by said rotor only after said engagementbetween said motion transmitting portion and said wall of said recessedportion.

6. The fluid pressure operated actuator defined in claim 1 furthercomprising a source of fluid pressure;

first directional control valve means adapted to connect said source offluid pressure to said pressure chamber to cause said piston means tomove in said onedirection; and

second directional control valve means operable simultaneously uponactuation of said first directional control valvemeans to communicatesaid source of fluid pressure to said one of said pair of ports torotate said rotor and thus rotate said member in a direction whichcorresponds to said one direction of reciprocal movement of said pistonmeans. I

7. The fluid pressure operated actuator defined in claim 1 when theproduct of the cosine of the pressure angle of said member and thetangent of the lead angle of said member is equal to or greater than thecoeffic'ient of friction of said engaged threaded portions.

1. A fluid pressure operated actuator comprising: a tubular housinghaving an internal bor enclosed at its opposite ends; piston meansaxially reciprocally disposed in bore tubular housing bore and dividingsaid bore into two pressure chambers; means for communicating pressurefluid to one of said pressure chambers while exhausting the other ofsaid pressure chambers to move said piston means in one direction insaid tubular housing bore; an actuating rod carried by said piston meansand extending from one side of said piston means through one enclosedend of said tubular housing; a member extending from the opposite sideof said piston means and rotatably supported in the other enclosed endof said tubular housing, said member having a threaded portion engaginga mating threaded portion of the actuating rod carried by said pistonmeans, the engaging threaded portion carried by said piston means andthe threaded portion of said member being in a self-locking relationshipsuch that said piston means is axially movable only when said onepressure chamber is communicated to said pressure fluid and said memberis rotated relative to said piston means; non-locking means for rotatingsaid member relative to said piston means, said non-locking means beingcarried by said other enclosed end of said tubular housing; and meanscoupling said non-locking means to said member such that saidnon-locking means and said member are rotatable together after a limitedamount of independent rotational movement of said non-locking meansrelative to said member; said non-locking means for rotating said memberrelative to said piston means comprising a rotor carried by said member,an enclosed chamber formed in said other enclosed end of said tubularhousing, said rotor hving a plurality of radially extending vane memberswhich divide said last mentioned enclosed chamber into a plurality ofsubchambers; and a pair of ports, one of which is adapted to directpressure fluid into said subchambers against said vane members to causerotation of said rotor and thus said member in one angular direction,the other said ports being adapted to exhaust fluid from saidsubchambers.
 2. The fluid pressure operated actuator defined in claim 1further comprising means for preventing relative rotation between saidpiston means and said tubular housing.
 3. The fluid pressure operatedactuator defined in claim 1 wherein said threaded portion carried bysaid piston means comprises a bore extending along the longitudinal axisof said actuating rod and opening to said opposite side of said pistonmeans, a portion of said bore being threaded a selected distance, saidrotatably supported member being in axial alignment with saidlast-mentioned bore and engaged therein.
 4. The fluid pressure operatedactuator defined in claim 3 wherein the product of the cosine of thepressure angle of the member and the tangent of the lead angle of themember is equal to or greater than the coefficient of friction of saidengaged threaded portions.
 5. The fluid pressure operated actuatordefined in claim 1 wherein said coupling means comprises: a circularsection formed on said member, said circular section having at least oneradially extending motion transmitting portion, said rotor having acircular bore receiving said circular section such that said rotor isangularly slidable about said circular section, said rotor having arecessed portion opening to said rotor circular bore, said recessedportion receiving said motion transmitting portion, said recessedportion being so sized with respect to said motion transmitting portionto permit said rotor to rotate about said circular section a limitedangular distance prior to engagement of a wall of said recessed portionwith said motion transmitting portion as said rotor is rotated when oneof said pair of ports is communicated to said fluid pressure, saidmember being rotated by said rotor only after said engagement betweensaid motion transmitting portion and said wall of said recessed portion.6. The fluid pressure operated actuator defined in claim 1 furthercomprising a source of fluid pressure; first directional control valvemeans adapted to connect said source of fluid pressure to said pressurechamber to cause said piston means to move in said one direction; andsecond directional control valve means operable simultaneously uponactuation of said first directional control valve means to communicatesaid source of fluid pressure to said one of said pair of ports torotate said rotor and thus rotate said member in a direction whichcorresponds to said one direction of reciprocal movement of said pistonmeans.
 7. The fluid pressure operated actuator defined in claim 1 whenthe product of the cosine of the pressure angle of said member and thetangent of the lead angle of said member is equal to or greater than thecoefficient of friction of said engaged threaded portions.